It is well known that many gas turbine engines employ a bleed strap which is opened as the engine is started. Essentially, a bleed strap is a mechanism which, when open, allows a portion of the compressed air from the compressor to bypass the turbine. As a result, the engine compressor operates in such a manner that its stall margin is increased. Safe and proper operation of the engine requires the bleed strap be closed near the end of the start sequence as the engine is approaching idle operation. Most jet engines automatically provide for bleed strap operation. However, few engine controllers have any provision for directly determining the status of the bleed strap.
Consequently, the aircraft pilot must perform an indirect check for bleed strap failure (open bleed strap) prior to take off. Traditionally, once the engine has started, and achieves normal idle operating conditions, the pilot throttles the engine to intermediate power and simultaneously checks the position of the exhaust gas nozzle. If the nozzle exceeds a threshold value (e.g. approximately 30% of maximum opening), it indicates to the pilot that the bleed strap has failed in an open position and requires maintenance.
However, newer digital electronic engine controls (DEEC) have control systems which compensate for bleed strap failure by increasing fuel flow to maintain engine schedules. This method of control prevents detection of a start bleed strap failure by simply monitoring the engine exhaust nozzle area.
It would be advantageous to have a gas turbine engine diagnostic system which would detect an engine start bleed strap failure from measured engine parameters independent of the exhaust nozzle area. The present invention provides such a system.